Field Notes: Moths on the Move

They gobble up leaves, damaging and sometimes killing trees in their path. Their invasion of Minnesota isn't a matter of if, but when—and it's already happening.

Realistically, foresters can only hope to slow down the spread and reduce the impact of the invading gypsy moth, one of America's most destructive nonnative forest pests.

This summer the Minnesota Department of Agriculture, in cooperation with the U.S. Forest Service and the DNR, is planning to employ a new trick—synthetic pheromones—to foil gypsy moths in northeastern Minnesota forests.

Each year, gypsy moth caterpillars defoliate about 880,000 acres of forest nationwide. They were brought to the United States from Europe in 1869 by a French scientist trying to breed a hardier silkworm. Some moths escaped, and 29 years later the first outbreak of the defoliating caterpillars occurred in Massachusetts. Since then, the gypsy moth has slowly moved westward.

Now Minnesota is on the front line of invasion. MDA traps in the state's northeastern forests began catching more gypsy moths in 1999. Numbers jumped from 192 gypsy moths trapped in 2004 to more than 1,000 in 2005—indicating that gypsy moths are likely breeding in the area.

To disrupt gypsy moth mating, the MDA has scheduled an aerial application of a synthetic gypsy moth pheromone in late July or early August across 135,000 acres in Cook County. Female moths emit a natural pheromone to attract male moths. The replica pheromone—contained in tiny green plastic flakes (about the size of a hyphen) spread at a rate of about two flakes per square foot—confuses male moths so that they cannot find mates.

The synthetic pheromone will only affect gypsy moths and won't disrupt other moth behavior. It won't affect fish behavior and is not known to affect aquatic invertebrates. It will not harm other wildlife or humans.

Most flakes will stick to trees; even those that fall to the ground will be nearly impossible to see. Treated areas will be identified by announcements in the media and at places such as state parks, so that people will know why planes are flying overhead.

DNR foresters say an infestation in Cook County could be the result of travelers unknowingly transporting moth eggs from infested areas farther east. For instance, campers might carry egg masses on vehicles or firewood.

Among the host species preferred by the gypsy moth, aspen and birch are abundant in Cook County. A risk model developed by DNR Forestry to predict tree mortality suggests that many birch trees are likely to be killed. If the pests spread south and west deeper into Minnesota, several other tree species—especially red oak—will be at risk.

That's alarming news to Minnesota's timber industry. Areas of infestation will be quarantined to prevent the transportation of the moth with nursery and forest products. Tourism could also suffer in areas with caterpillar-covered trees and piles of caterpillar droppings and shed skins.

Without treatment, gypsy moths spread by windblown larvae at a rate of about 13 miles per year. Pheromone treatments elsewhere in the United States have been shown to slow the spread to six miles per year. A slowdown after treatment could give the DNR and its partner agencies time to reduce potential effects on forest and tourism industries. It could also give forest managers time to bolster the health of tree stands so they can better withstand an onslaught of gypsy moths.

Gypsy moths spread most quickly by hitchhiking rides with humans. If you're traveling from an infested area, such as Wisconsin, Michigan, or New England, be sure to check your vehicle and equipment for egg masses between May and July. Remove and burn any you find, or soak them in water or kerosene. Never bring firewood from infested areas to state parks or forests. If you find gypsy moths, call the Arrest the Pest Hotline at 888-545-MOTH.

Jean Goad, DNR information officer